Recently, researchers led by Prof. Yan Jun and Prof. Li Chaolun of the Center for Ocean Mega-Science, Chinese Academy of Sciences made progress in in-situ Raman spectroscopy quantitative detection of a deep-ocean hydrothermal fluid system. With the homemade Raman insertion probe (RiP), they conducted in-situ quantitative Raman spectroscopy detection of high-temperature hydrothermal fluids in the hydrothermal field in the middle Okinawa Trough. They obtained in-situ concentrations of dissolved CO₂ and SO₄^2- in the high-temperature hydrothermal vent fluids for the first time in the world. Their research findings are published as the cover paper in Geochemistry, Geophysics, Geosystems.

A major discovery of earth science in the 20th century, the deep-ocean hydrothermal fluid system creates matter and energy exchanges between different circles.  In recent years, the physical and chemical properties of high-temperature hydrothermal vent fluids and their influence on the ocean have become new research highlights. Temperature, pressure changes, and seawater mixing can significantly alter the chemical composition or concentration of hydrothermal vent fluids. Although scientists have obtained relatively close data by laboratory analysis with a gas-tight sampler, they cannot obtain correct samples of the high-temperature hydrothermal vent fluid due to the restriction of sampling methods, which cause obvious differences between analysis results and the reality. The research team has successfully developed the first high-temperature (450℃) resistant RiP (Xin Zhang et al., DSR-I, 2017) in the world, by solving the effect of heat-resistant and high-concentration particle optical lens on the optical system. By means of research vessel (RV) Kexue and remotely operated vehicle (ROV) Faxian, the RiP has detected lots of high-temperature hydrothermal vent fluids in the hydrothermal fields in the Manus and the Okinawa Trough since 2015 and collected a number of in-situ Raman spectra.

Depending on the in-situ Raman spectra (up to 273℃) of three high-temperature hydrothermal vent fluids in hydrothermal fields in middle Okinawa Trough obtained by RV Kexue’s comprehensive voyage for hydrothermal fluids and cold seep in 2016, and the numerous Raman spectroscopy quantitative analysis models of CO₂ and SO₄^2- (Lianfu Li, Xin Zhang*, et al., Applied Spectroscopy, 2018; Shichuan Xi, Xin Zhang*, et al., Applied Spectroscopy, 2018) set up by high-temperature simulation experiments, the researchers have determined the concentrations of CO₂ and SO₄^2- (Lianfu Li, Xin Zhang*, et al., G-cubed, 2018) of hydrothermal vent fluids in the middle Okinawa Trough. According to the research, the content of SO₄^2- as seawater mixing index in measured high-temperature hydrothermal fluids is nearly zero. It proves that the hydrothermal fluids sampled by the RiP contain no seawater, that is to say, the measured samples can be seen as primitive hydrothermal vent fluids. By comparing the CO₂-concentration in the same hydrothermal vent measured by ROV with gas-tight sampling, it is clear that the concentration determined by in-situ measurement is three times higher than that determined by gas-tight sampling in a laboratory. Based on the results, we can say that the influences of hydrothermal fluids on global carbon cycles and climate change may be much greater than people thought. The findings are of great significance in promoting the in-situ spectroscopy detection in the extreme deep ocean environment, and may help people re-understand the influences of hydrothermal fluids on global oceans.

The research was funded by the National Natural Science Foundation of China, the Ocean Pilot Project of the Chinese Academy of Sciences and the Frontier Science Key Programs of the Chinese Academy of Sciences. PhD student Li Lianfu is the first author of the paper, and Prof. Zhang Xin is the corresponding author.

For more information, please contact:

Prof. Li Chaolun

E-mail: lcl@qdio.ac.cn

Source: Center for Ocean Mega-Science, CAS